Human vessels includes arteries, veins, tracheae, bronchi, esophagi, bile ducts, urethras, etc, the diameters of which vary from several millimeters to a few tenths of millimeters. Those human vessels may have various diseases, for example, collapse, occlusion and damage, and corresponding endoluminal stents may be used for treating those endoluminal diseases. Those endoluminal stents should satisfy some common requirements. For example, they may be encapsulated into tiny sheathes, delivered into human vessels and then released, expanded and then supported at the predetermined endoluminal positions, for the purpose of treatment.
As arteries are more likely to have diseases than other human vessels, endoluminal stents are most commonly used for treating arterial diseases. There are many kinds of arteries, typically circumferential arteries, mainly including carotid artery, subclavian artery, lower extremity artery, renal artery, etc. Arterial stenosis or occlusion is mainly caused by atherosclerosis, arteritis, fibromuscular dysplasia, etc. In western countries, about 90% arterial stenosis diseases are caused by atherosclerosis, and in China, arteritis is the common cause of arterial stenosis.
Carotid artery stenosis may result in ischemic syndromes in the head and eyes, for example, dizziness, headache, syncope, temporary amaurosis, blindness, etc. Sudden onset of subclavian arterial occlusion is usually followed by sudden limb pain, skin temperature drop, skin color change and even limb amputation if severe; and, subclavian arterial stenosis is usually followed by weakness, numbness, cold of limb, and intermittent limb pain after activities. Renal arterial stenosis is likely to result in renal arterial dysfunction. Sudden onset of lower limb arterial occlusion will result in sudden limb pain, skin temperature drop, skin color change, pale skin, and even limb amputation if severe; and lower limb arterial stenosis is usually followed by weakness, numbness, cold of limb, and intermittent limb pain after activities.
At present, there are the following methods for treating arterial stenosis.
For patients suffering from slight arterial stenosis or patients having no clinical symptoms, medical drug therapy may be adopted which, however, does not work for patients with severe symptoms. The physician will make a therapeutic scheme according to the situation of a patient, including drugs for regulating lipid and dropping blood pressure. Some patients may further need to use anticoagulant drugs to reduce the danger of causing thrombus in stenotic arteries by the blood.
For those suffering from severe arterial stenosis or complete occlusion, surgical treatment may be adopted. For an artery having a diameter of 4 to 18 millimeters, the surgical treatment is bypass operation, i.e., arterial bypass grafting. That is, the diseased stenotic arterial segment is bypassed and normal vessels at its two ends are connected by a new path. However, surgical operation causes big injuries and has many complications.
Arterial thrombolysis and thrombectomy are applicable to acute iliofemoral artery thrombus or embolism patients, however, are not applicable to chronic vessel stenosis or occlusion lesions.
For transluminal coronary angioplasty, Dotter and Judkins first conducted percutaneous angioplasty by a coaxial nylon balloon dilating catheter in 1964, which was the beginning of balloon percutaneous transluminal angioplasty (PTA). With the development of transluminal angioplasty, minimally-invasive therapy is being increasingly applied. Different from the traditional operations, transluminal coronary angioplasty has become a main treatment method for arterial stenosis or occlusion as it causes minimal injury and offers quick postoperative recovery.
At present, there are mainly two types of transluminal coronary angioplasties, one of which is balloon percutaneous transluminal angioplasty (PTA), and the other is stent implantation. Balloon percutaneous transluminal angioplasty is to expand the diseased vessel by a balloon in order to dredge the vessel. In spite of good effects in the short term, balloon percutaneous transluminal angioplasty has a high incidence of complications mainly including plaque or detachment of thrombus which causes embolism of distal vessels. Additionally, the incidence of vessel restenosis after angioplasty is high. Stent implantation is to implant a vascular stent at the diseased vessel position to expand the vessel at the diseased position. After the stent is implanted, there is a certain radial support which reduces the possibility of vessel restenosis. At present, there are mainly two kinds of stents for circumferential vessels; i.e., balloon-expandable stents and self-expanding stents. For a balloon-expandable stent, it is expanded by a balloon and has a certain radial support force due to plastic deformation. Such a stent, although accurate in positioning due to small axial shortening after expansion, is poor in its flexibility and thus is just applicable to be placed in a flat vessel; furthermore, the incidence of restenosis is higher than for self-expanding stents. For a self-expanding stent, it can automatically return to its original shape by virtue of its own super-elasticity and shape memory characteristic after being released from the sheath. Self-expanding stents have been widely applied due to their excellent performance and fewer complications, and are more applicable to vessels on the limbs (which are often in bending motion) than balloon-expanding stents. In the prior art, self-expandable stents for circumferential vessels are almost entirely made of nickel-titanium canals by laser engraving, and only a few of the self-expandable stents are braided from nickel-titanium alloy wires.
At present, self-expandable endoluminal stents are almost always laser-engraved stents; that is, are stents formed by laser-engraving nickel-titanium alloy tubes and then thermally treating those tubes for shaping. Self-expanding endoluminal stents manufactured in this way are not applicable to be bent in human endoluminal positions having activities to some extent, due to their poor flexibility and high occurrence of fatigue fracture.
There are self-expandable endoluminal stents braided from nickel-titanium wires, and self-expanding endoluminal stents braided from nickel-titanium wires are applic able to be placed at diseased and bent human endoluminal positions having activities to some extent due to their excellent flexibility and fatigue resistance. However, the braided self-expandable endoluminal stents in the prior art are quite high in axial shortening; that is, after an endoluminal stent previously compressed in the sheath is released from the sheath, the length of the endoluminal stent after it automatically expands is significantly shortened. Such shortening will result in difficulties in the positioning and control of the stent during the release process; and, the radial support force of the stent is reduced and the stent is likely to experience displacement.